The present invention relates to a method for the production of a color filter layer system structure on a substrate, in which a structured lacquer layer with lacquer layer surface regions and lacquer-free regions is deposited in lift-off technique, subsequently a color filter layer system is deposited and, lastly, with the lacquer layer surface regions the regions deposited thereon of the color filter layer system are removed.
Such a technique is known as lift-off technique. It is relatively cost-efficient since therein relatively few process steps have to be carried out in a vacuum atmosphere. This technique is known, for example, from U.S. Pat. No 3,914,464.
As a lift-off mask a photosensitive lacquer or a metal layer with given thickness of, for example, 4 to 6 xcexcm is deposited and thereover is deposited a dielectric interference color filter layer system of only one half the thickness. The lift-off lacquer is cured over a long period of time, more than 8 hours at 200xc2x0 C. in a vacuum atmosphere, before the color filter layer system is precipitated above it. In the lift-off step proper the lift-off lacquer or its structured areas are removed with hot xylene after precipitation of the color filter layer system.
It is evident that the lift-off lacquer layer structure is treated at high expenditures and specifically such that vapor deposition can even be used as the precipitation process for the color filter layer system. In order to ensure satisfactory quality of the precipitated system layers, a vapor deposition temperature of more than 200xc2x0 C. must be maintained. The subjacent lacquer layer must, in any event, be able to tolerate these temperature without undergoing any changes.
Furthermore, it is known from the literature, for example H. A. MACLEOD, xe2x80x9cThin-Film Optical Filtersxe2x80x9d, Second Edition, Adam Hilger Ltd., 1986, pp. 357 to 405, that optical layers vapor-deposited at lower temperatures are spectrally unstable. Their spectral properties vary with ambient temperature and humidity, with the cut-on and cut-off edges of the spectral characteristics, in particular, being shifted. If the intent is to precipitate spectrally stable layers with satisfactory quality, the vapor-deposition temperature must be very high which leads to the fact that the subjacent lacquer is polymerized and the subsequent lift-off process is made extremely difficult. On the other hand, if it is attempted to keep the temperature of the vapor-deposition process low, this, as described, is obtained at the expense of strong spectral shifts of said edges, which shifts are a function of the temperature and air humidity.
The long heating of the lacquer in vacuo, proposed in the above cited publication, is, in addition, extremely expensive and reduces significantly the production rate of production installations for such structures. The use of hot xylene for dissolving the lacquer resistant to temperatures, which, by necessity, are relatively high, furthermore is highly questionable in view of the carcinogeneity of xylene. Since, moreover, due to the method of applying the color filter layer system at temperatures far above 200xc2x0 C., a lacquer layer resistant to these temperatures must be provided, large attack areas for the solvent must be made available for the subsequent removal of the lacquer Therefore, the lacquer layer thickness must be significantly greater than the thickness of the color filter layer system to be deposited thereon. This results in large disturbed zones; if, following the lacquer application and the structuring of the lacquer layer, a color filter subsystem is deposited in the lacquer regions and those free of lacquer, then, as depicted in FIG. 1, due to the shadowing by the edges of the lacquer regions 1, disturbance zones result in the margin regions 3 of the color filter structure, in which the thickness of the color filter structure is less than in the undisturbed regions 5. The greater the lacquer thickness d, the greater are the regions of the disturbed color filter structure.
It is, in addition, known that substrate glasses shrink, i.e. change their form, at the temperatures necessary for the vapor deposition of said color filter layer systems. This leads to the fact that the precipitation of color filter layer systems taking place side by side, with differing spectra, such as, for example, for red, yellow and blue, cannot take place with geometric precision which leads to lack of sharpness between filter regions of differing spectral regions.
The glass substrate, furthermore, becomes brittle at said high temperatures, in particular taking into account the alternating thermal stress, for example during the sequential precipitation of several color filter layer systems in lift-off technique. This leads to degradation of the stability against fracture of the manufactured structures and therewith also to an increase of disturbed structures.
It would be a further alternative to use a lacquer which endures the requisite high vapor deposition temperatures, thus temperatures in the range of 300xc2x0 C. However, such lacquers are expensive and difficult to process further.
Further, lift-off masks of metal, for example comprising Al or Cr, could be used. Since their application, however, conventionally requires again a vacuum coating step, this would also be too expensive.
Building on a process of the above cited type, it is the task of the invention to remedy said disadvantages. This is attained when carrying out said process according to the invention.
It becomes thereby possible to stress thermally the lift-off lacquer to a significantly lesser degree during the precipitation of the filter layer systems, that is, far below the polymerization temperature of, for example, conventional photosensitive lacquers. Through the proposed plasma-enhanced precipitation process, in which the substrate is exposed to high ion bombardment density, preferably through plasma-enhanced vapor deposition or through sputtering, furthermore filters with dense layers and spectral properties are generated which are temperature and humidity stable. Since the lacquer is thermally significantly less stressed, its necessary thermal resistance can be decisively reduced which, in turn, leads to the fact that a significantly shorter lacquer treatment and significantly reduced lacquer layer thicknesses are required which, in turn, reduces the extension of the disturbed regions 3 according to FIG. 1. Moreover, conventional photosensitive lacquers can be used and can be dissolved by means of customary solvents, such as by means of acetone or NMP (N-methyl-2-pyrrolidone) for the lift-off.
The process according to the present invention
consequently results in significantly reduced coating temperatures which leads to the reduction of the necessary lacquer thickness and to the fact that conventional cost-effective lacquers can be used. The further resulting lacquer thickness reduction leads directly to the reduction of the extension of the disturbed regions according to FIG. 1.
Due to the plasma enhancement of the coating, be that by generation of charged particles in a separate plasma chamber, extraction through grids into the coating volume or by plasma generation in the coating volume, a bombardment of the substrates with an ion current of high density is realized, with which dense, optically stable filter layers are realized.
The extension of the disturbed loci effected by shadowing can be reduced through additional directing measures, such as provision of collimators or through electrostatic orientation of the ion movement. In addition, low minimum pressures are used in the coating chamber, with correspondingly large mean free path length.
If, without providing additional directing measures, sputtering is used, according to FIG. 1, disturbed regions 3 with an extension of approximately 5* d are obtained, with more directed plasma-enhanced processes down to approximately 1* d. This results even with the values for d which, according to the invention, are low per se.
A more directed coating process than sputtering is the plasma-enhanced vapor deposition in which, as is known, a material is thermally vaporized from a vaporization crucible into the process atmosphere, for example by electron beam vaporization, wherein a plasma is maintained in the atmosphere. Therewith the problem of the shadowing is further remedied, and it becomes also possible to precipitate color filter layer system structures with very small extension under control and with high efficiency.
The proposed process is especially suitable for the color filter structuring for LCD projectors and CCD sensors.